This invention relates to production of magnetic elements, and particularly to the formation of magnetoresistive elements for use in a transducing head or the like. More particularly, the invention relates to detection of multiple magnetic domains and of the stability of magnetic domains in magnetic elements being formed on a wafer.
Magnetoresistive (MR) sensors are responsive to changes in magnetic fields by changing resistance. Such sensors are increasingly employed as read transducers in the heads of magnetic disc drives, primarily because the change of resistance in the sensor is independent of disc speed and depends primarily upon changes in the magnetic flux. These sensors typically comprise a thin strip of NiFe alloy (Permalloy) of low coercivity, with an easy axis of magnetization along the strip. Many other ferromagnetic materials are also candidates, although Permalloy is the most commonly used. Recently, the strips have been mounted on the head between permanent magnet wings to stabilize the head to a single magnetic domain. Additionally, soft magnetic layers (SALs) have been placed adjacent the strips to further stabilize the head.
One problem encountered with MR sensors is Barkhausen noise caused by the irreversible motion of magnetic domains in the presence of an applied field. It is known that Barkhausen noise is eliminated by creation of a single magnetic domain in the sense current region of the MR element. However, multiple magnetic domains may be formed during fabrication of the MR element. It is, therefore, important to be able to identity, MR elements having multiple magnetic domains.
The patterning of permanent magnets on a wafer involves ion milling into or through the Permalloy, deposition and lift-off of the permanent magnet film, and patterning of the Permalloy and permanent magnet films that form the sensor structure. Finally, the active area is defined by the contacts. Substantial processing problems in any one of these steps, or a combination of minor problems in more than one of these steps, can cause the formation of a multiple domain sensor. For example, pinning sites at the contact edges or insufficient stabilizing field on the permanent magnets can lead to multiple domain sensors. More particularly, the stabilizing effect of the permanent magnet wings of a permanent magnet stabilized MR head is considered to be sensitive to process parameters. An inadequately stabilized film could, for example, be formed by too thin of a permanent magnet film or the placement of permanent magnets too far from the sensor, or formation of an active region having too high or too low of a height to achieve reliable repeatability. The performance of the head could also be affected in other ways related to the presence of a multiple-domain sensor. It is, therefore, important to be able to identify MR elements having multiple magnetic domains or which destabilize in an external magnetic field.
Previously, MR elements had been sorted through inspection in a Kerr effect microscope by visually determining when the domains changed under rotation of the magnetic field. MR elements have also been sorted by examining the elements using magnetic force microscopy. Both inspection methods require expensive equipment not normally used in production, and both are labor intensive and tedious. Furthermore, these processes are expensive and time consuming, and the use of the Kerr effect microscope is also inaccurate for very small elements. Transfer curve testing, which is quick and inexpensive, has been used to detect Barkhausen noise, but transfer curve testing requires saturation of magnetic shields adjacent the element to avoid generation of inaccurate data.